This invention relates to a connection for drill pipe and more particularly, pin and box connectors disposed on the ends of tubular members for connecting the tubular members for the drilling of bore holes in the pipeline and utility industry.
Drill strings are specifically designed for the installation of telephone lines, fiber optic cable, sewage lines, water lines, and similar installations relating to utilities for the pipeline and utility service industry. The typical objective is to drill a bore hole from point A to point B, generally under some type of obstacle or structure for the installation of a utility. Typically the bore hole distance drilled is between 200 and 600 feet. Also typically the bore hole is shallow and is generally horizontal.
In a typical drilling operation, a bit, such as a fishtail bit, is mounted on the end of the drill string with a transmitter located just behind the bit. The drill string is placed in compression and rotated to rotate and apply force on the bit to drill the bore hole. Fluids are circulated through the drill string and bit and then back up the annulus formed between the wall of the bore hole and drill string to moisten the earth through which the bit is drilling. This lessens the tendency for the drill string to stick in the bore hole. A hand-held receiver is located above ground to follow the transmitter and determine the orientation of the bit, i.e. inclination and azimuth. The operator then adjusts the orientation of the bit if the bit varies from the trajectory of the proper bore hole path between points A and B.
Once the bore hole is drilled under the obstacle, then the bit is removed and a reamer on a swivel is attached to the end of the drill string and the drill string and reamer are pulled back through the bore hole with the utility attached. The swivel is attached to the utility line such as a cable for example. The drill string is placed in tension as the reamer is pulled back through the bore hole with the utility attached.
The operator minimizes the amount of fluid that is circulated through the drill string because too much fluid into the surrounding earth bore is undesirable. If too much fluid is circulated through and around the drill string, the fluid begins to wash out the sand, dirt, and earth causing a washout underneath the obstacle or structure, such as a highway or waterway, under which the bore hole is being drilled. Structural problems will be created if a void is formed underneath a highway or some other structure.
Drill strings for the pipeline and utility industry are much different from that used in the oilfield. In oil field drilling, a drill string is used to drill a wide open bore hole. Drilling fluid is circulated to remove cuttings and cool the bit. Thus, it is important that the connections in the drill string be sealed from pressure and fluid. Typically it is not important whether there is fluid leakage through the connection in a drill string for the pipeline and utility industry. Thus the connections between joints of drill pipe are not required to seal against pressure or fluids and heat and pressure are not of concern. The only pressure is the pump pressure for flowing drilling fluid.
When designing the drill string and other related tubular goods for a particular drilling application, not only must the physical orientation of the drill string be taken into consideration, but also the desired use. The drill string may be placed in a horizontal orientation, or in a vertical orientation, or in a deviated position. The actual orientation depends on the type of application.
The successful drilling of bore holes in the pipeline and utility industry requires the assembly of many individual tubular members into a drilling string. The actual assembly entails the make-up and torquing of individual tubular members. The actual drilling process involves multiple make-ups and breakdowns of the connections between individual tubular members of the drilling string. The number of makes and breaks depends upon the environment and the operator. Typically, a connection will be made up and broke out four or five times a day and could be as high as six or seven times a day. Thus, a connection between joints of pipe may have a 1,000 or more makes and breaks a year and therefore must be robust and rugged to withstand such use. Also, the assembly is performed very quickly since time considerations are very important.
Design considerations for the connections for the drill string joints include, but are not necessarily limited to, the repetitive coupling and uncoupling of the connections, torquing, bending, cyclic loading, fatigue, tensile loads, and compressive strength. The connection typically includes threaded pin and box connections. The threads must be capable of making up easily with other threaded joints. The longevity of the individual thread is also very important for several reasons. First, once a thread begins to deform, further deformation is usually exponentially increased thereafter. Second, once the thread has deteriorated, the entire drill string must be disregarded since the length of the drill string has very limited variance. Third, failure of the thread while the drill string is in the process of actually drilling a bore hole will cause substantial monetary damage. The preceding list is meant to be only illustrative.
Despite these designs, when a drill string member has a thread that is used in multiple applications for the drilling of bore holes in the utility and pipeline industries, the mating shoulders may become flared due to over torquing. Also, the prior art designs result in the thread profile becoming deformed which in turn causes the external and internal thread design profiles to lose their make-up capability and results in premature failure. Once deformation begins, continued use exponentially increases mating problems between cooperating tubular members. Further, the pin and box are unscrewed, and thereafter, when made up again, the deformation forces will only be enhanced. In other words, the multiple screwing and unscrewing of the connections accelerates the deformation process so that once the deformation begins to occur, the process will thereafter exponentially increase.
FIG. 1 illustrates a prior art connection and the inherent problem with multiple fastening and unfastening of prior art connections in the utility industry due to over torquing and cycling of the box and pin. The prior art connection typically includes a pin end 150 for mating with a box end 152. The pin end 150 includes an annular shoulder 154 which is perpendicular to the axis of the connection. A round thread external thread profile 156 extends from shoulder 154 to an outer diameter 158. A radial terminal end 160, also perpendicular to the axis of the connection, extends from outer diameter 158 to an inner diameter 161.
The box end 152 includes a radial terminal end 162 having a surface which is perpendicular to the axis of the connection. A round thread internal profile 164 extends from terminal end 162 to an internal surface 166. A radial shoulder 168 extends from internal surface 166 to the internal diameter 170 of the tubular member.
As shown in FIG. 1, the radial terminal end 162 has slid on shoulder 154 and become deformed. One cause for this deformation may be the over torquing of the box 152 and pin 150 ends whereby the radial end 162 has been forced to slide outwardly on the face of shoulder 154 of the pin end 150. The forces thus applied may also deform the radial end 160 of the pin 150 such that the radial end 160 has been deformed, and in particular, by the outer surface 158 and the matching inner surface 161 being expanded outward by the compressive action of the radial shoulder 168 being forced into the radial end 160.
The pin and box are cut on a two inch cone taper. When the pipe string is driven, the outside cone of the box as the string is placed in compression, the outside cone of the box tends to drive outward at the base of the pin. Thus you are tight at the small end and loose at the large end when placed in compression. Thus, as the connection is placed under stress and incurs a cycling motion, the connection fatigues and breaks. The tubular members rocking on each other cause the connection to become loose.
The over torquing situation will also cause the thread profiles to no longer match properly. Thus, as shown in FIG. 1, a first gap 172 and a second gap 174 emerges. Of course, while FIG. 1 represents two gaps 172, 174, other gaps along the thread profile may in fact occur.
Typically prior art connections include standard API threads 176, 178 on the pin end 150 and box end 152, respectively, shown in FIG. 1A. API threads have positive flank angles on both the stab flanks 151 and load flanks 153. U.S. Pat. No. 5,154,452 discloses a tubular connection for the oilfield having a S-shaped thread profile providing a reverse angle thread form to withstand severe tension placed on the connection during drilling. However, the connection has very little compression capability.
Cyclic bending, also known as rocking, i.e. lateral forces allow the pin end 150 to move or slide relative to the box end 152, is another cause of the flaring so as to form gaps 172, 174. Ideally, there should be no abrupt change in the direction of drilling. However, many applications, such as deviated settings, produce several abrupt dog legs or kinks in the borehole due to the operator making abrupt changes in the direction of drilling. Abrupt changes in drilling trajectory causes tortuosity which is a term describing a borehole which has a twisted trajectory. Tortuosity substantially increases the cyclic bending, torque and drag on the drill string.
As the pipe rotates and passes through a bend or an arch in the trajectory of the bore hole, the drill string bends causing the mating surfaces of the connection to slide back and forth particularly at the rotary shoulders. The smaller the radius of the bend in the bore hole, the greater fatigue that will be placed on the pin. As the drill string becomes deviated and twisted, the rocking within the thread profile will be exacerbated. As the deformation increases, the rocking can only escalate.
As the pipe string passes around obstacles in the drilling path, it passes through steep bend radiuses. As the pipe string passes through the multiple severe bend radiuses, a bend is placed on the connection causing the threads to disengage on one side of the connection. As the connection passes through other bends, the connection bends in the other direction.
As the connection passes through these severe bends, the positive flank angles on the API threads 176, 178 act as ramps causing the threads to further engage causing the connection to get tighter. The positive thread flanks of the API thread allow rocking of the connection because the threads work into the roots. As the connection is rocking, there is a ramping action between the threads. This ramping action causes a yielding of the thread material and the thread member is stretched. The ramping of the threads cause yielding of the material and thus damage the connection. In other words, the thread material passes beyond its point of elasticity eventually causing the pin to be broken off. Rocking causes fatigue and pin breakage.
Once the reamer is connected to the end of the drill string and is being pulled back through the borehole, high tension is applied to the drill string and particularly the connections. As the reamer engages obstacles in the drilling path, bends and torque are placed on the connections causing the threads to open and close. As the threads are stretched apart, the connection is further tightened due to the torque placed on the string.
Each time the drill string is rotated once, it is called a cycle. The drill string rotates three to four hundred cycles per minute during the drilling operation. Each time the connection passes through a bend and rotates, the shoulders slide back and forth against each other. These repetitive cycles with sliding shoulders provides a continuing fatigue at the mid-portion of the pin. This causes substantial stress and causes the pin to break off at the connection. The pin tends to break near the base of the pin and is typical in the prior art joints. Once the pipe is rotated through an arch or bend in the bore hole, the terminal end of the box tends to slide away from the base of the pin. It tends to come apart as the two faces slide against each other. This causes the threads near the shoulder of the pin and the terminal end of the box to come apart. However, the connection continues to stay tight at the mid-portion of the threads. With the mid-threads holding tight and the shoulder of the pin and terminal end of the box rocking, the pin tends to break at the base. During one half of the cycle the pin is bent one way and then during the other half of the cycle it is bent the other way causing it to fatigue and break. This back and forth action causes tremendous fatigue. The pin typically breaks after a few thousand cycles. A corkscrew bore hole places peculiar bends and cycles on the drill string which may well double the bend radius capacity of the joint. All of these factors induce rapid fatigue in the connection.
Also, during the drilling process, a large amount of compressive force is placed on the drill string which tends to drive the terminal box end outwardly as the surfaces' slide. The greater this force, the greater the gap between the base of the pin and the terminal end of the box. However, the mid-portion of the threads remain tight. Then at every cycle, the counter-stretching back and forth begins to fatigue the connection.
If the environment is very bad and if you have an inexperienced operator, the operator tends to over-compensate and steers too hard. An inexperienced operator also makes a direct turn with the drill string. Once the inexperienced operator realizes that he has turned the drill string too much, then he makes an abrupt turn in the other direction. This causes a tortuosity in the hole being drilled. This places the drill string in a very serious bind as it is rotated through the bore hole. The number of cycles that the connection will last is diminished considerably.
Another of the problems of the prior art connection is its ability to withstand high torque. The operator often over torques the connections. An extreme amount of torque is applied to the connection as compared to the small diameter of the pipe causing a large amount of torque to then be applied to a small diameter drilling string. Further, as the pipe string passes through severe multiple bends, high inertia torque is placed on the connections as the pipe string passes around obstacles and forms severe multiple bends. Thus, the connection must also withstand high inertia torque.
In the oil field, the drilling operators are much more experienced and the distances being drilled are much greater. Also, more sophisticated equipment is used so that the drilling is more controlled. Further, experts are continuously monitoring the drilling. In the utility industry, often the operators are inexperienced and force the drill string to achieve a bore hole from point A to point B and do not drill a smooth bore hole. Operators in drilling utility lines are more interested in getting the job completed than they are the wear and tear on their equipment.
It is preferred to have no movement between the surfaces because when there is movement, fatiguing occurs. Once the surfaces begin to slip, then the stretching of the pin and box begins. The pin can absorb some stretching during the cycling of the drill string. If the stretch occurs within limits, fatigue does not set in. Once the stretching exceeds the limit, then the connection begins to fatigue and will break.
The connection of the present invention solves these and other problems and deficiencies of the prior art as will be more clear from the description of the advantages, features and embodiments that follow.